1. Field of the Invention
The present invention relates to the use of low alloy steels in applications involving anti-coking properties. Such steels arc primarily intended for the manufacture of the elements of apparatus such as reactors, furnaces or conduits or for coating such apparatus primarily used in refining processes or in the petrochemicals industry, such steels having an improved resistance to coking.
The invention also relates to novel compositions of such steels with an improved resistance to coking.
The carbonaceous deposit that develops in furnaces when converting hydrocarbons is generally termed coke. This coke deposit is a nuisance in industrial units. The formation of coke on the walls of tubes and reactors causes a reduction in heat exchange, major blockages and thus an increase in pressure drops. In order to keep the reaction temperature constant, it may be necessary to increase the temperature of the walls, which risks damaging the alloy constituting those walls. A reduction in the selectivity of the facilities, and thus a reduction in yield, is also observed
2. Description of the Prior Art
Japanese application JP-A-03/10 4843 describes a refractory anti-coking steel for an ethylene cracking furnace tube. However, that steel comprises more than 15% of chromium and nickel and at least 0.4% of manganese. That steel was developed to limit coke formation between 750xc2x0 C. and 900xc2x0 C. when cracking ethylene.
In addition, U.S. Pat. No. 5,693,155 concerns petrochemical processes using low coking stainless steels. Such steel contain at least 10% of nickel and at least 10% of chromium.
Surprisingly, it has now been discovered that the enrichment in silicon of low alloyed chromiumxe2x80x94molybdenum steels has a beneficial effect on the anti-coking properties of said low alloyed steels. Because of the chromium and nickel contents lower than those of the steels disclosed in U.S. Pat. No. 5,693,155, the steels of the present invention are less expensive.
An object of the invention is an apparatus comprising furnaces, reactors or conduits having at least their internal walls, on which coke can appear, composed at least partly of a steel with a composition which is adapted to resist coking comprising, by weight, at most 0.25% of C, 1.5% to 5% of Si, 4% to 10% of Cr, 0.5% to 2% of Mo, 0.3% to 1% of Mn, at most 0.03% of S and at most 0.03% of P, the complement to 100% being essentially iron. Such apparatus are intended for refining or petrochemical processes carried out at temperatures of 350xc2x0 C. to 1100xc2x0 C, particularly catalytic naphtha reforming reactions, carried out at temperature, of 450xc2x0 C. to 650xc2x0 C. and isobutane dehydrogenation, carried out at temperatures of 550xc2x0 C. to 700xc2x0 C.
A further object of the invention is a steel composition for use in these applications comprising, by weight, at most 0.25% of C, more than 2.5% and up to 5% of Si preferably more than 3.1% Si, more than 7% to 10% of Cr, preferably at least 7.1% to less than 10.0%, 0.5% to 2% of Mo, 0.3% to 1% of Mn, at most 0.03% of S and at most 0.03% of P, the complement to 100% being essentially iron.